1. Field of the Invention
The invention relates to UV additives for amino resins with a nitroxyl compound soluble in an aqueous aminoplastic precondensate, to a UV-resistant amino resin, to a UV-resistant aminoplastic molding composition, to a UV-resistant aminoplastic precondensate, to a process for producing a UV-resistant laminate, to a process for preparing UV-resistant amino resins, and for preparing UV-resistant aminoplastic molding compositions.
2. Description of the Related Art
The excellent properties of thermosets, especially amino resins, gives them outstanding suitability for many applications where demands are, inter alia, scratch resistance, flame retardency, mechanical stability, gloss, environmental compatibility and chemical inertness. From these applications, resistance to UV radiation over many years is also needed, since yellowing or yellow coloration of the surfaces is undesirable. Amino resins therefore require addition of suitable UV stabilisers. Very high requirements are placed upon these UV stabilisers suitable for amino resins. They include, for example, good solubility in the amino resin, since a precondition for their effectiveness is homogeneous distribution of the UV stabilisers in the resin matrix. Another criterion is chemical stability during the curing process characteristic of aminoplastics. Another decisive factor in that there has to be lasting binding of the UV stabiliser within the aminoplastics. This means that there must, for example, be no subsequent leaching of the stabiliser out of the aminoplastics. Another decision factor is that UV protection is active immediately, i.e. as soon as mixing with the aminoplastics has ended. A further decisive factor is that the UV stabilisers do not color aminoplastics but ensure that transparency continues over the years.
U.S. Pat. No. 5,629,426 prepares sterically hindered nitroxyl compounds of 4-hydroxy-2,2,6,6-tetramethylpiperidine type from the underlying 4-hydroxy-2,2,6,6-tetramethylpiperidine, which is not a free radical, via an oxidation process with the aid of hydrogen peroxide in the presence of a carbonate or bicarbonate of ammonium or of an alkali metal as catalyst. One possible use of these nitroxyl compounds is the use as polymerisation inhibitor to prevent premature polymerisation of vinyl monomers.
U.S. Pat. No. 5,436,345 describes another preparation method. An application sector given is the polymerisation of reactive acrylic monomers.
EP 0581737 B1 again describes the use of sterically hindered nitroxyl components of 2,2,6,6-tetramethylpiperidine type. They are described as polymerisation inhibitors in the monomer preparation process for aromatic vinyl components (examples being styrene, alpha-methylstyrene, or in other substituted vinylbenzenes).
Compounds of the type represented by ortho-hydroxyphenylbenzotriazoles, 2-aryl-2H-benzotriazoles, hydroxyaryl-1,3,5-triazines or sulfonated 1,3,5-triazines are also known and are used in textile materials as absorbers for the UV-A and UV-B components of sunlight, these being injurious to the skin. According to EP 0659877 A2, the sun-protection factor of clothing can be increased by adding these compounds to the rinsing water for a textile material, for example. The UV absorbers are in aqueous solution, or if water-solubility is poor, in disperse form when incorporated into the textile material.
There are no UV additives known hitherto which fulfil the requirements for amino resins. The properties of the additives known for thermoplastics are inadequate for aminoplastics. EP 0704560 B1, for example, describes a synergistic combination of UV absorbers from the ortho-hydroxyphenylbenzotriazoles group with a sterically hindered amine (HALS, hindered amine light stabiliser) from the 2,2,6,6-tetramethylpiperidine compounds group, for UV protection of thermoplastic fibres. These UV stabilisers cannot be used for aminoplastics, since they cannot be incorporated homogeneously into aminoplastics.
Furthermore, they have the disadvantage that the substance responsible for the UV protection has to be formed in situ, not until exposure to UV light has begun. This means that there is a time delay in the protective action with respect to UV irradiation.
The object of the present invention was to provide a UV additive for amino resins.